The present invention is concerned with an optical array amplifier, particularly, but not exclusively, for use in optical telecommunications systems and networks employing wave division multiplexing (WDM). More specifically, the invention concerns method and apparatus for achieving amplification of optical signals and also optical signals amplified by the method or apparatus.
Optical transmission systems and networks have become increasingly more accepted as a preferred mode for carrying communications traffic over large distances. One particular mode of optical communication that has gained acceptance recently involves the multiplexing of large numbers of wavelengths onto a single fibre. In this way, the bandwidth of an optical fibre carrying communications traffic can be massively increased, implying a vastly improved capacity for traffic.
Such systems are generally known as Wave Division Multiplex (WDM) systems. Point-to-point, long-haul systems are currently proposed in which 160 or even up to 640 wavelengths may be employed. Such systems require wavelength division multiplexers/demultiplexers and optical amplifiers to ensure proper transmission between nodes throughout the system or network.
In order to enhance the flexibility of these systems, there is a need to allow a range of operations to be made possible. For example, it is advantageous, if not necessary, for individual wavelengths or groups of wavelengths to be added or dropped at intermediate nodes in a network and for so-called xe2x80x9cexpressxe2x80x9d wavelengths to bypass certain nodes.
Systems providing these capabilities are complex to design, principally because the demultiplexing, filtering and switching components that are needed to split and route wavelengths are xe2x80x9clossyxe2x80x9d and therefore require the use of amplification to compensate for this loss. To complicate matters further, some of the wavelengths may require significantly different amplification, depending on factors such as whether they are to be added, dropped or bypassed at the node in question, and the distance travelled before the node or to be travelled after the node. Some of these may be in conflict with one another and can add significantly to the cost of designing the system.
Known ways of overcoming these problems have been ineffective in practice and/or ineffective in cost terms. For example, perhaps the obvious way of addressing per wavelength amplification in a WDM network is literally to demultiplex the wavelengths and to use a separate amplifier for each. This approach carries an inordinate cost penalty. Each wavelength or group of wavelengths would require a complete optical amplifier, including pump, WDM multiplexer, VOA, Erbium-doped fibre and controller.
An alternative approach that attempts to share some of these components would involve placing the amplifier upstream of the WDM demultiplexer. All wavelengths would then be amplified simultaneously and to approximately the same extent. However, this arrangement would not necessarily generate the high signal output powers that are often required to overcome the lossy optical components that are in use at, for example, an optical flexibility node. The approach is clearly less flexible than one in which each wavelength is processed individually, in terms of amplification and equalisation.
A further approach involves the use of a compact array of semiconductor optical amplifiers (SOAs). Such arrays are smaller and of lower cost than comparable arrays of Erbium-doped fibre amplifiers (EDFAs) but exhibit a poorer performance.
However, none of these techniques satisfactorily resolves the problems identified above and does not offer an acceptable compromise in terms of cost and performance.
The above and other disadvantages are overcome by the present invention which, in its broadest sense, provides an optical amplifier offering a cost effective means of achieving per-wavelength amplification. In its preferred form, it eliminates large numbers of passive devices and drastically reduces fibre handling. The amplifier of the invention also provides a building block that can be integrated with other devices, such as taps, variable optical attenuators, opto-electronics and electronics in such a way as to enable highly integrated subsystems to be created.
In a first aspect, the invention provides an optical array amplifier comprising:
a first light beam shaper;
a first waveguide adapted to guide an input optical signal consisting of a plurality of wavelengths to said first beam shaper;
a diffracter adapted to be illuminated by light from said first beam shaper whereby to diffract said optical signal into individual wavelengths or groups of wavelengths thereof,
a second waveguide adapted to be connected to a source of optical pump energy;
a second beam shaper adapted to combine optical pump energy from said second waveguide with said diffracted signal wavelengths or groups of wavelengths without said pump energy impinging on said diffracter, such that a proportion of said optical pump energy is mixed with each said wavelength or group of wavelengths, and to focus the mixed optical signals onto a third waveguide adapted to be connected to an optical amplifier utilising said pump energy to cause amplification of the individual wavelengths or groups of wavelengths mixed with the corresponding proportion of pump energy.
In a second aspect, the invention provides a method of amplifying an optical signal consisting of a plurality of wavelengths, the method comprising:
guiding said optical signal through a first waveguide to a first beam shaper;
shaping said optical signal in said first beam shaper so as to direct said optical signal onto a diffracter;
diffracting said optical signal by said diffracter into individual wavelengths or groups of wavelengths thereof;
mixing optical pump energy provided through a second waveguide from a source thereof with said diffracted signal wavelengths or groups of wavelengths without said pump energy impinging on said diffracter, such that a proportion of said optical pump energy is mixed with each said wavelength or group of wavelengths; and
directing the mixed optical signals through a second beam former onto a third waveguide adapted to be connected to an optical amplifier utilising said pump energy to cause amplification of the individual wavelengths or groups of wavelengths mixed with the corresponding proportion of pump energy.
The array amplifier may be implemented as a planar waveguide array or as a bulk optical device. There may be one or more pump sources. More than two pump energy waveguides may be provided, their position in the second beam shaping region being tailored to suit the required output characteristics. Sources of pumping energy may be co-located on the waveguide array package. The beam shaping elements may be mirrors or lenses or regions of a planar waveguide, the diffractive element may be a bulk diffraction grating or a waveguide array or these elements may be combined into a grating element which also provides the beam shaping, such as an xe2x80x9cEchelle gratingxe2x80x9d.
Amplification may be effected in Erbium-doped fibres or waveguides connected to the third waveguides. The outputs may be attenuated in dependence on the amplitude of the output signals. The power level of the one or more pump sources may similarly be controlled. The phase and/or amplitude of the pump source may also be controlled, for example by mode shaping resulting from variation in the cross-section of the pump waveguide(s).
By way of specific example, the invention can be based on planar integrated optics using an Arrayed Waveguide Grating Wavelength Division Multiplexer (AWG-WDM). The waveguide amplifiers can be based on planar integrated optics or optical fibres, with the optical amplification being by means of doping with, for example, Erbium, or by alternate non-linear optical means, such as Raman amplification.
The optical signals are preferably wavelength modulated optical communication signals.
The invention also provides a gain flattening filter, incorporating an optical amplifier as defined in the above paragraphs. The invention yet further provides an optical add/drop device incorporating an optical amplifier as defined in any of the previous paragraphs.
The invention will be described with reference to the following drawings, in which:
FIG. 1 is a schematic representation of a xe2x80x9cstandardxe2x80x9d arrayed waveguide wave division multiplex arrangement;
FIG. 2 is a schematic representation of a first embodiment of the invention;
FIG. 3 is a schematic representation of a second embodiment;
FIG. 4 is a schematic representation of a third embodiment;
FIG. 5 is a schematic representation of a fourth embodiment;
FIG. 6 is a schematic representation of a fifth embodiment
FIG. 7 represents schematically a typical optical pumping arrangement;
FIG. 8 is a schematic representation of an application of the invention;
FIG. 9 is a schematic representation of an implementation of the invention;
FIG. 10 is a schematic representation of a further implementation of the invention;
FIG. 11 is a typical gain response curve for an Erbium-doped fibre; and
FIG. 12 is a schematic representation of an implementation of the invention incorporating a control mechanism.